CA1153798A

CA1153798A - Water-resistant, high-voltage cable

Info

Publication number: CA1153798A
Application number: CA000359364A
Authority: CA
Inventors: Hermann-Uwe Voigt; Cornelius Van Hove
Original assignee: Kabelmetal Electro GmbH
Current assignee: Kabelmetal Electro GmbH
Priority date: 1979-08-31
Filing date: 1980-08-29
Publication date: 1983-09-13
Also published as: DE2935224A1; EP0025091A3; JPS5636806A; AU6087480A; EP0025091B1; EP0025091A2; ATE41075T1

Abstract

ABSTRACT OF THE DISCLOSURE
A cable is disclosed in which, preferably, a polyethylene insulation contains grafted-on silane which is cross-linked in the armorphous regions on-ly to form water-repelling siloxane groups; the PE crystals are substantially free from cross-linked points, but contain grafted-on silane as immobile water-treeing inhibitors.

Description

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The present invention relates to a waterproof, high-voltage insulation for electrical cable. More particularly, the invention relates to an olefin polymerisate or olefin copolymer, elastomeric polymers, and thermoplastic rubbers, individually or as a blend, and to be inhibited against generation and growth of so-called water trees.
Water damage to the insulation of electrical cable is a problem of long standing. The damage results from ingress of water whereby so-called water trees are produced; i.e., locally the insulation becomes defective due to water, and the defects have the appearance of a tree. Many investigations have been conducted to find and trace the specific causes for these defects.
Many of these investigations have been published, referring particularly to the growth of water trees and often including suggestions to avoid this problem with the goal in mind to increase the life span of such a cable. Generally, it is agreed upon that the accumulation of water is the more dangerous the more extensive it is. Trees having but a few micrometers in length are deemed comparatively harmless.
The safest way to avoid water trees, but also the most expensive one, is to provide a metal jacket around the cable in such a manner that any welded joint is watertight with certainty. Additionally, the formation of any internal ~0 channels is avoided.
Concerning various attempts to avoid the growth of water trees, United States Patent 4,145,567 describes a cable in which the insulation is covered by a semiconductive shield which, in turn, carries a compressed layer of closed-pore-type foam. This foam layer is encased by a metal strip being bonded along overlapping edges. This dual moisture barrier may be effective, but is rather expensive and increases the radial dimensions of the cable; also, bonding the edges does not ensure permanentimpermeability to moisture.

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German printed Patent Application 27 54 336 discloses a different approach; this application proposes the generation of a diffusion gradient.
For example, the insulation is surrounded by a water-absorbing layer. This arrangement does not take into consideration that water trees are particularly prone to develop in interfaces such as between insulation and conductive layers or elements.
German printed Patent Application 28 17 804 suggests a power cable in which a layer containing a hygroscopic material is provided outside the outer semiconductive layer. This layer limits the relative humidity that can reach the insulation up to 70 percent. Allegedly, this feature protects the insulation even if the insulation includes impurities and voids. Related to this approach is the cable disclosed in United States Letters Patent 4,029,830, in which the insulation itself includes water-absorbing filler material.
Another approach suggests the inclusion of a water-soluble electrolyte~ All of these approaches, however, can only have the effect of delaying damage to the cable when generating an electrical field. The inclusion of these supple-mental substances has, however, the disadvantage that they themselves consti-tute insulation impurities, and that may give rise to problems at higher volt-ages.
~0 United States Letters Patent 4,144,202 follows a different approach for avoiding water trees in insulations. This patent suggests the inclusion of an epoxy containing an organic silane, in an ethylene polymer, to serve as water-treeing inhibitor. Tests reported in this patent demonstrate the effect-iveness of this approach. It has been found, however, that additives to such an insulating material as used in cable-engineering and including, e.g., sta-bilizers, antioxidants, softeners, etc., tend to diffuse out of the rather thin insulation so that their effectiveness deteriorates. Again, this kind of additive can only dela~ but not impede the generation o~ Water trees.
Enti~ely independent from the foregoing, siloxane cross-linking is known in which an olefin polymer or copolymer elastomer or thenmoplastic rubber is cross-linked through grafted-on silane of the general form~la .format R Si Y3, wherein R is a vinyl group or a y-methacrylate propyl group and Y is an alkoxy group with less than six carbon atoms. ~ec for example, United States Patents 3,646,155; 4,048,129 and 4,058,583.
None of these patents, however, relates to water-treeing. The known silane cross-linking relates to strength enhancement of the particular material.
The present invention provides for a new and improved protec-tion of the insulation of a cable as against moisture generally and the formation and growth of water trees in particular, avoiding thereby the various problems out-lined above, particularly in regard to cable oonstruction and undesired side effects. ~owever, mDre should be done that just delaying the formation of . water trees.
: In one aspect, the present invention provides moisture-pr~of~ hiyh-voltaye cable, which co~prises a core and an insulation, the insulation compris-ing a base material being an olefin polymer or copolymer, an elastomeric co~pound, or a thermDplastic rubber, singly or in a blend; and a silane compound of the type R Si Y3, wherein R is a vinyl group or y-~.ethacryloxypropyl yroup and Y is an alkoxy group having less than 6 car-bon atoms, said silane compound being grafted on the base material and being substantially free of cross-linking in crystalline regions of the base material.
In a further aspect, the present invention provides a method of mak ing a hiyh-voltage insulating elect.rical cable under conditions establishing an ~;37~

inhibition again~t the ~orm~tion of watex tree~ co~,prising the steps of grafting the silane-grafted polymer or copolymer to a conductor as a layer at a temperature above the crystalline melting point; and causing the applied silane-grafted polymer or copolymPr to solidify under conditions which exclude cross-linking during the crystalliæation of the applied silane-grafted polymer or copolymer, so tha-t all or a significant amount of the grafted but uncnoss-linked silane remains in the layer.
In accordance with a preferred embodiment of the present invention, it is suggested to graft a silane or silane compound upon the macromolecules of the polymer or copolymer constituting the ba æ material for the insulation.
The silane or silane compound is to serve as inhibitor for the formation of water trees. The base material m~st prominently used is polyethylene. m e preferred silane compound is a vinyltrimethoxysilane, a vinyltriethoxysilane or a similar vinyl silane having hydr~lizable alkoxy groups in lieu of methoxy or ethoxy groups. me silane content should be at least approximately 0 2% of base material, and not more -than 10%, preferably, one should use from 1% to 3%
by weight. The silane-grafted material is then applied to the conductor or cable co~e, e.g., by means of extrusion, which is followed hy permitting the compound to dry, i.e., crystalline in the absence of any curing that involves the silane. Particularly, any cross-linking must be inhibited until the in-ternal crystal structure has been established. Subsequently, the insulation will be exposed to moisture, intentionally or uninten-tionally, so that at least some cross-linking and curing may occur, especially if the initial blend in-cluded a condensation catalyst. This cross-linking, particularly in the interior of the insulation, is limited to the amorphous regions in ~he insulation. The polymer crystals will contain and retain the uncross-linked, grafted-on silane.
~he invention is based on the follGwing aspects. It is ~elieved tlla~

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wa~er treeing occurs only ln the amorphous portion of the insulation or, more particularly, in the interface and boundary zone between amorphous and crystal-line regions. The crystallites have a higher density and resist more any charge carriers which are accellerated in the electrical field that penetrates the insulation. Moreover, these crystallites are nearly impermeable to water.
Thus, any water that tends to penetrate (diffuse) the insulation must bypass the crystallites. Consequently, a decline in crystallinity generally enhances the probability of water diffusion.
It was found that a significant amount of cross-linking (regardless of the mechanism involved, peroxidic, by means of radiation, steam) induced prior to crystallization results in the generation of cross-linking bridges which constitute embedded impurities and, thus, reduce the degree of crystal-linity in the insulation upon cooling. This then renders the insulation sen-sitive to water treeing.
On the other hand, inhibiting the cross-linking of silane-grafted polymer until after a highly crystalline, internal structure has been obtained, limits subsequent cross-linking (intentionally or unintentionally), i.e., the formation of cross-linking proper as between the silane, to the relatively small amorphous regions. The crystalline structure itself remains unaffected and does not, or will very little, cross-link. Cross-linked points are general-ly "weak" points on which water trees may form. As a consequence, the insula-tion is composed of a highly crystalline structure which is impermeable to moisture, and the grafted-on silane acts as water-treeing inhibitor. The poly-functional cross-linking points, being clustered in the small amorphous re-gions, increase the strength of the solidified material. Both effects, a high degree of undisturbed crystallinity and cross-linking points in the amorphous portions of the insulation, increase the life of the insulation and produce the _ 5 _ 7~

overall resistance against later treeing, which remains constant throughout.
Critical for a lasting effect is the fact that the silane inhibi-tor is chemi-cally bonded to the macromolecules and, thus, i~mobilized; it will not diffuse out of the material. Even at elevated temperatures and under the influence of an electrical field, the grafted-on silane stays in place.
It is important that -the silane is effective, regardless of any sub, sequent cross-linking. It should be noted that grafting of silane is kno~n per se follcwed by cross-linking through exposure to mQisture (see, e.g. Canadian Patent Nos. 1,057,888 and 1,078,545 of one of us)~ This enhan oe s resistance against temperature and mechanical stn~ngkh. Presently, it is decisive that the mDis-ture cross-linking mechanism ke impeded until the material has solidified under a high degree of crystallinity. Then, and only them, should cross-linking occur, in the residual amphorus regions.
It is interesting in this regard that the siloxane group Si-0-Si is actually hydrophobic. rrhis retards regular as well as electrophoretical dif-fusion, the latter occurring when an AC-field is set up in the insulati.on.
Thus, any water does either participate in the siloxane cross-linking (and will not form trees) or is directly repelled by the hyd~ophobic properties imparted upon the insulation by any completed link.
r~he advantages and features of the invention will be better under-stood from the following description taken in connection with the accompanying drawings, in which:
the figure shows a cross section thL~ugh a cable in accordance with a preferred emb~di~ent of the invention, made upon practicing a preferred m~de thereof.
The cable includes a multifilament conductor 1, carrying a smDothing layer 2 made of conductive ~aberial and being provided in orler lo av3id ~orma-~.~.ei 7~3~

tion of high, local fields. An insulation layer 3 is disposed on top of layer

2. Layer 3 is, for example, made of a silane-grafted polyethylene at a highly crystalline state. The grafted-on silane may not be cross-linked because its compound did not include a condensation catalyst, or it solidified in a dry environment so that the formation oE crystals was not interfered with by cross-linking The polymer of layer 3 is cross-linked only in the amorphous regions.
Insulation jacket 3 is covered by an outer, conductive shield 4 which, in turn, is covered by an armoring 5 comprised of wires. An outer jacket 6 made, e.g., of polyvinyl chloride is provided as protection of the cable against mechanical damage.
The cable is made in the following manner~ The base material, e.g.
polyethylene, is mixed with a silane or silane compound, homogenized followed by grafting, e.g., at a temperature from 180C to 250C. This blending and grafting may be produced in a combined process involving an extruder. The grafting is followed by applying the blend to the conductor assembly 1, 2. For example, the first-mentioned extruder feeds a second extruder which has a head through which a hose-like cover is laid onto the passing conductor assembly 1, 2 tCascade technology). The transition construction by means of which the first extruder feeds the second extruder may include a degassing stage. Basic-ally, one can use equipment as shown in United States Letters Patent 4,117,063.
The immediate use of the grafted polymer in the application extruder is not essential as long as cross-linking of the material is positively inhibited.
This inhibition must continue beyond the application as a jacket around the conductor 1, 2 until the material has solidified in a crystal structure. The formation of a large crystal structure must not be impeded by cross-linked points and the residual amorphous zones are quite limited in size. Any expo-7~

sure to moisture n~ust occur only thereafter so that cross-l:inking will be re-stricted to these amorphous zones.
The following ~wo examples were found to be suitable for the inven-tive purpose:
Example I
Polyethylene homopolymerisate 100 parts (Density: 0.918 to 0.930; melt index 0.2 to 2.5) Vinyltrimethoxysilane 1.8 to 2.3 parts 1.3-bis (tert. butyl peroxy isopropyl) benzene 0.02 to 0.1 parts Dibutyltindilaura~e 0.05 to 0.1 parts (all parts by weight) Example II
Polyethylene copolymerisate with less than 100 parts 8 mol % vinyl acetate or acrylate ~Density: 0.93 to 0.935; melt index 2.0 to 8.0) Vinyltrimethoxysilane 1.5 to 2.0 parts Dicumyl peroxide 0.05 to 0.1 parts Dibutyltindilaurate 0.05 to 0.1 parts (all parts by weight) A blend of these components is fed to the mixing and grafting ex-truder as mentioned above and homogenized therein and grafted in a single step;
the application to the cable core may immediately follow in a continuation of that one step; degassing may be included, as was mentioned above. Alternative-ly, one may provide grafting in a first stage; and the grafted material is blended with a batch for the cross-linking catalyst, just prior to feeding the silane-grafted PE to the application extruder.
In order ~o pro~e effectiveness of the invention, a plurality of ~3~

samples were prepared. One group had an insulation of regular polyethylene (PE); another group was provided with insulation of a polyethylene cross-linked in steam (CPE); and a third group was made as per the present invention, the insulation was of silane-grafted polyethylene (PE-Sil.).
The insulation was rated in each insta-nce for transmission of 20 kilo-volts, and these cables were aged under operating conditions (20 kv), different temperatures, and for 200 hours each. Thereafter, the conductors 1 and shield 5 were filled with water; and the conductors were cyclically heated, and ex-posed to a median field strength of 6 kv/mm for 120 days. The residual strength o the thus treated cables was ascertained under particular alternating volt-ages in a one-hour step test.
The following table illustrates the result of the composite test.
Insulation Type PE CPE CPE PE-Sil PE-Sil . ._ _ I
Aging temperature 90 90 120 90 130 in cen-tigrades Example I:

Voltage step in kv 60 76 76 120 109 Exposure time in minutes 7 5 6 13 2 Example II:

Voltage step in kv 87 87 76 98 120 Exposure time in minu-tes 5 7 2 2 2 _ , , , ,_ . ._ . . . .. ... . . . . .
This table shows that regular PE as well as cross-linked PE, aged under usual conductor temperatures, are about equally resistant against water, but grafted-on silane reaches much higher vol-tage steps.
The invention is applicable to all electrical cables to be operated at higher voltages ~20 kv and up~. The invention is not limited to the ~mbodi-~3~

ments described above; but all changes and modifications ~hereof, no~ consti-tuting departures from the spirit and scope of the invention, are intended to be included.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of making a high-voltage insulating electrical cable, under conditions establishing an inhibition against the formation of water trees com-prising the steps of grafting silane or a silane compound onto an olefin polymer or copo-lymer;
applying the silane-grafted polymer or copolymer to a conductor as a layer at a temperature above the crystalline melting point; and causing the applied silane grafted polymer or copolymer to solidify under conditions which exclude cross-linking during the crystallization of the applied silane-grafted polymer or copolymer, so that all or a significant amount of the grafted but uncross-linked silane remains in the layer.

2. The method as in claim 1, wherein the polymer or copolymer does not contain, nor has added thereto, a condensation-type cross-linking catalyst.

3. The method as in claim 1, including the subsequent step of cross-linking the applied silane-grafted polymer or copolymer of exposure to moisture so that the silane cross-links in amorphous regions to form hydrophobic siloxane groups.

4. The method as in claims 1, 2 or 3, wherein the grafting is carried out by heating the silane or silane compound and olefin polymer or copolymer to a temperature of between 180° and 250°.

5. Moisture-proof, high-voltage cable, which comprises a core and an insulation, the insulation comprising a base material being an olefin polymer or copolymer, an elastomeric compound, or a thermoplastic rubber, singly or in a blend; and a silane compound of the type R Si Y3, wherein R is a vinyl group or y-methacryloxypropyl group and Y is an alkoxy group having less than 6 carbon atoms, said silane compound being grafted on the base material and being substantially free of cross-linking in crystalline regions of the base material.

6. The cable according to claim 5 wherein the grafted-on silane compound is cross-linked in amorphous regions of the base material.

7. The cable according to claim 5 wherein the grafted-on silane compound is substantially free of cross-linking throughout the base material.

8. The cable according to claims 5, 6 or 7 wherein the insulation com-prises from about 0.2% to about 10% by weight silane.

9. The cable according to claims 5, 6 or 7 wherein the insulation com-prises from 1% to 3% by weight silane.